A common process for producing an optically active amine is the process wherein a nitrogen functional group is introduced in an optically active alcohol by substitution reaction (See, for example, C. P. Chen et al., Tetrahedron Lett., 32, 7175 (1991); A. S. Thompson et al., J. Org. Chem. 58, 5886 (1993), etc) This process, however, is associated with a safety problem due to the use of an azide compound which suffer from the risk of explosion, and as a consequence, the control of the production process was complicated.
In view of such situation, various production processes have been proposed wherein an optically active amine compound is directly produced from the corresponding imine by an asymmetric reducing reaction. (For a general review, see, for example, Shohei HASHIGUCHI et al., Journal of Synthetic Organic Chemistry Japan, 55, 99 (1997)). A known process is the process wherein an imine is asymmetrically hydrogenated by using a rhodium, iridium or titanium complex having a chiral ligand. This process, however, required use of hydrogen atmosphere at a pressure as high as 40 to 130 atm., and the preparation of the catalyst was quite complicated.
Another process is the hydride reducing process wherein an imine is reduced by using an optically active metal hydride comprising a metal hydride complex compound such as lithium aluminum hydride or sodium borohydride or a metal hydride such as diborane modified with an optically active protonic compound. Exemplary such process is asymmetric hydride reduction process using an optically active hydride modified with an optically active alcohol, amine, or amino alcohol. This process, however, requires the use of an equivalent amount of the optically active compound.
In view of such situation, many investigations have been conducted to develop a hydride reducing process wherein the asymmetricity source of only catalytic amount is required. For example, in a process using oxazaborolidine complex, an asymmetric reducing reaction using a catalytic amount of the oxazaborolidine complex is realized as in the case of the asymmetric reducing reaction of a ketone. However, when the amount of the asymmetricity source, namely, the complex is reduced from the equivalent amount to the catalytic amount in this process, the resulting product suffers from significantly reduced optical purity. In addition, use of the borane-sulfide complex for the reducing agent also resulted in the complexity of the process, requiring countermeasures for safety, odor, and the like.